DIMERIC ASSEMBLY OF ENTEROCYTE BRUSH-BORDER ENZYMES

The noncovalent, dimeric assembly of small intestinal brush border enzymes was studied by sedimentation analysis in density gradients of extracts of pulse-labeled pig jejunal mucosal explants. Like aminopeptidase N (EC 3.4.11.2), sucrase-isomaltase (EC 3.2.1.48-10), aminopeptidase A (EC 3.4.11.7), and dipeptidyl peptidase IV (EC 3.4.14.5) were all observed to dimerize predominantly prior to the Golgi-associated complex glycosylation, i.e., in the endoplasmic reticulum or in an intermediate compartment between this organelle and the Golgi complex. However, small amounts of monomeric complex-glycosylated forms, in particular of sucrase-isomaltase, were detectable. This indicates that homodimerization cannot be an absolute requirement for transport to, and through, the Golgi complex although our data suggest that dimeric assembly may increase the rate of intracellular transport. Culture at low temperature (20 degrees C) reduced the rate of, but did not prevent, dimerization. Maltase-glucoamylase (EC 3.2.1.20) only appeared as a dimer when extracted and analyzed under low salt conditions, suggesting a weak association between the two subunits. This finding is consistent with the electronmicroscopic appearance of the liposome-reconstituted enzyme [Noren et al. (1986) J. Biol. Chem. 261, 12306-12309], showing only the inner, membrane-anchored domains of the monomers to be in close contact with one another while the outer domains are far apart. In contrast to the other brush border enzymes studied, lactase-phlorizin hydrolase (EC 3.2.1.23-62) was found to occur predominantly as a monomer in its transient, high mannose-glycosylated state. Dimerization mainly took place after complex glycosylation but before; or simultaneously with, the proteolytic cleavage(s) generating the 160-kDa subunit of the mature enzyme.